The early process of vertebrate neurogenesis is divided into several basic processes, such as differentiation into the neural plate (neural induction) and formation and maturation of the neural network from the ectoderm. This early process includes the appearance of neural precursor cells, pattern formation of the nervous system, and proliferation and differentiation of neural precursor cells. For the understanding of the molecular basis of higher brain functions, it is important to elucidate the universal, underlying principles of these processes in all species.
It is known that the early neurogenesis of Xenopus laevis is induced by blockade of BMP4 (Bone Morphogenetic Protein 4) signals by noggin, chordin, etc. secreted by the organizer [Sasai, Y. et al.: Nature, 376:333 (1995)]. BMP4 is a factor which induces the ectoderm into epidermal cells; under a condition where BMP4 is activated, cells differentiate into the epidermis.
On the other hand, as genes involved in the control of neural induction (neurogenesis, neural differentiation) which are called proneural genes, Neurogenin, NeuroD, XASH-3, XATH-3 and the like coding for basic-helix-loop-helix (bHLH) transcription factors are known.
However, it is still unknown what regulates the cascade linking the blockage of BMP4 signals to proneural genes.